The present invention relates to an improved method for quickly cooling post solid-stating reaction and post crystallizer standard flakes, small particle flakes, or pellets of recycled or virgin polyethylene tetephthalate (PET), from temperatures typically in the range of 380xc2x0 F. to 410xc2x0 F. to temperatures of less than 150xc2x0 F., without causing significant change in molecular weight or moisture carryover in the cooled PET.
PET, as well as other polymers, co-polymers, and polycondensates, whether virgin, post-industrial, or post-consumer, in the form of small particle flakes (4 mm or less in the major axis), standard flakes (xc2xcxe2x80x3 to xc2xdxe2x80x3), or pellets, that have been solid-stated to increase molecular weight, either in vacuum or in an inert atmosphere such as nitrogen gas, remain in a reactive state subject to further condensation polymerization or oxidation degradation and remain at a relatively high temperature (380xc2x0 F. to 410xc2x0 F.) at the end of the useful phase of the solid-stating process.
In batch solid-stating reactors, the polymer material must be cooled to a temperature below which further polymerization or oxidation reactions are eliminated. Only after the solid-stated polymer is cooled to a temperature where further reactivity is eliminated can it be safely and effectively transported out of the reaction vessel for further processing or packaging utilizing traditional material handling techniques. The cooling step in the traditional batch solid-stating process is accomplished in situ, under reduced temperatures, but always under vacuum or an inert gas atmosphere. Typically, the cooling period can take as much time as the temperature ramp-up and solid-stating polymerization phase. In order to make more efficient use of the reactor, and increase the productivity, a method for effectively cooling the reacted polymer is desirable.
It would be desirable to develop a process for cooling post reaction PET flakes in a manner that prevents a significant change in molecular weight and prevents chromophore formation.
A process for cooling post solid-stating reactor PET flakes, in a manner which prevents any significant change in molecular weight, has surprisingly been discovered. The process comprises the steps of:
conveying PET flakes from a solid-stating reactor to a first separator, in the presence of air and water at an elevated pressure, wherein the PET flakes are partially cooled and the water is converted to steam by contact with the PET flakes;
separating the steam from the partially cooled PET flakes; and
conveying the partially cooled PET flakes from the first separator to a second separator, in the presence of air at an elevated pressure, wherein the PET flakes are further cooled and dried.
The inventive process is particularly well-suited for cooling PET flakes which have been solid-stated to a particular intrinsic viscosity value which must be retained, for use in subsequent forming operations.